The existing industrial era dominated by mass production is giving way to a new era or flexible manufacturing. Flexible manufacturing is a key manufacturing strategy for modern industries, in which complex shaped structural components are prevalent but quantity requirements are low, resulting in high unit costs and long lead times.
Rapid prototyping, or layered manufacturing is perhaps one of the most significant new technologies which enables the full realization of the power of modern Computer Aided Design (CAD) and Computer Aided Engineering (CAE) tools. CAD and CAE provide the concurrent design and analysis of a component in order to obtain the optimal structural layout and details about its material composition.
The materialization of such optimal designs requires unconventional manufacturing processes, such as layered manufacturing. This integrated product and manufacturing process development is especially critical in the modern industries such as aircraft, airspace, automotive and medical implants and prosthetics, to name a few.
The demand for an affordable, high strength, lightweight complex components, capable of operating at elevated temperatures and in adverse environments, requires continuous efforts in advanced structural designs, new materials and manufacturing processes.
To meet these requirements, materials such as Titanium and continuous fiber Titanium Matrix Composites, which have stable high specific strength properties at elevated temperatures, must be implemented, and the fabrication technology to incorporate these materials into lightweight structural concepts must be developed.
The stumbling block toward the wide use of these materials has been the very high cost of the existing manufacturing methods used for the production of components from such materials. For example, titanium is difficult to machine and requires fabrication processes such as the Superplastic Forming and Codiffusion Bonding (SPF/DB) of titanium sandwich structuring. These fabrication processes require high tooling costs and long lead time.
Even more complex and expensive multi-step methods are used for the production of MMC components, such as foil/fiber layering with High Isostatic Pressure (HIP) consolidation. Very high capital and running costs are typical for these manufacturing processes.
Rapid Prototyping or Free Form Fabrication is present one of the fastest developing manufacturing technologies in the world.
Free Form Fabrication through layered material deposition has proved to be an attractive method for 3D object generation. The layered part buildup offers the possibility of expanding the implementation of advanced designs with respect to part complexity, multi-layer multi-material structures, and the optimal material distribution within the desired part.
Although significant improvements in application of high temperature structural materials have been achieved with such processes as SLS, Thermal Spray Shape Deposition and 3D welding, a number of inherent processing limitations prevents them from producing parts with structural integrity and accuracy readily obtainable by conventional manufacturing methods.
One of the main shortcomings of Thermal Spray Shape Deposition and 3D welding methods is the requirement for the liquefaction of the feedstock material prior to its spay or cladding deposition, which results in high energy deposition, large residual stresses, poor mechanical properties, layer debonding, and part distortions. In the case of metal powder sintering, the main shortcomings are the limited choice of specially prepared multiphase metal powder materials with limited physical and mechanical characteristics. The fabricated parts have low density and require a time consuming and expensive postprocessing. The part accuracy is unpredictable due to significant shrinkage after postprocessing.
In the Sheet Lamination Process as applied to metal layers deposition, the process is limited to layers joining by bonding or soldering. Attempts to use welding as a layer joining method proved to be difficult.
None of those processes lends itself to the production of continuous fiber MMC materials.
Extensive research efforts for the development of Rapid Prototyping technologies capable of producing metal and composite materials are being conducted within academic institutions and industry throughout the world. The USA leads the research community in this field, but in the last several years the research conducted in Europe has intensified, particularly in the area of rapid prototyping in metals. Several rapid prototyping processes, such as Thermal Spray Shape Deposition, 3D welding, Sheet Lamination Process and especially SLS, have progressed in the application of metal materials.
Originally, the process of Selective laser Sintering (SLS) was developed by the University of Texas at Austin. DTM Corporation has developed this process into a multi-step called RapidTool process that is capable of producing metal parts. It is a time consuming process for molding making using plastic coated iron based metal powder. Current research is being conducted on two-phase metal powder material such as Bronze-Ni.
SLS's main shortcomings are the limited choice of specially prepared multiphase metal powder materials with limited physical and mechanical characteristics. The fabricated parts have porosity and require a time consuming and expensive postprocessing. Part accuracy is unpredictable due to significant shrinkage after postprocessing.
Other related work is Thermal Spray Shape Deposition, 3D Welding and Laser Cladding.
Carnage Mellon University uses an arc-spray system in which two melted wires are sprayed onto a substrate to generate a part layer.
The University of Nottingham in the U.K. uses a MIG welding torch which melts a 0.8 mm feed wire with a current of 100 A and deposits it in the shape of a bead of weld that is approximately 4.5 mm wide and 1.4 mm thick. Using this technology, these dimensions represent the resolution of the system.
IPT in Germany is working on a metal part generation process with laser cladding which involves directing a high power laser (700 W CO) onto the substrate, and simultaneously melting the powder or wire feedstock at the laser spot.
One of the main shortcomings of Thermal Spray Shape Deposition, 3D welding and Laser Cladding methods is the requirement for the liquefaction of the feedstock material prior to its spray or cladding deposition, which results in high heat buildup, large residual stresses, poor mechanical properties, layer debonding, and part distortions.
SLS as well as 3D Printing processes have the promise of mixing fiber reinforcing particulates into a powder mix (SLS) or into ceramic slurry (3D printing), thus enabling the production of MMC or CMC components.
None of those processes lends itself, though, to the production of continuous fiber reinforced MMC materials which dominate military applications because of their superior performance/cost characteristics.